With the initial release of the USNO's Robotic Astrometric Catalog (URAT1), astronomers now have precise positions for about 228 million stars in the northern sky.

Given my love of stone walls, I've been thinking about building one in the backyard to enclose the flower beds. But I haven't had the gumption to actually buy the rocks and get started, because I know that it's going to take a long time to finish. Worse, I fear that I might undertake all that work, only to decide a few years later that I want to redo the wall using different stones.

That gives you a sense of what was going through the collective heads of astronomers at the U.S. Naval Observatory when, in 2012, they started a three-year effort to create a new catalog of all the stars in the northern sky down to declination –15°. What's more, they'd just wrapped up the fourth and final version of a star catalog they'd been working on since 1997. What were they thinking??

M11 URAT1-UCAC4 animation
Here's a comparison plot of the Wild Duck Cluster, Messier 11, as recorded with UCAC4 (the USNO's previous survey) and the just-released URAT1 survey. Their respective magnitude limits are 16.5 and 19.0. The circle is about 15 arcminutes across.
U.S. Naval Observatory

"Advances in detector technology" is what. The previous effort, known as the USNO CCD Astrograph Catalog (UCAC), had coupled a tack-sharp 0.2-meter astrograph, known as the "Red Lens," with a 16-megapixel Kodak CCD detector. After long observing stints on Cerro Tololo in Chile and at Flagstaff, Arizona, the end result was all-sky coverage to roughly magnitude 16½, about 113 million stars in all, with positions pinpointed to within 70 milliarcseconds (0.07 arcsecond) or better. But "the actual images from the UCAC program look terrible," notes Norbert Zacharias of the USNO's astrometry department, with lots of electronic noise from dark current in the CCD chip.

So in 2012 Zacharias and his team commissioned a new survey using the same telescope but a much better detector. This one boasts 110 megapixels (10,560 by 10,560), covers about 28 square degrees at a time, is cooled by liquid nitrogen, and records stars to at least 18½ (two magnitudes fainter). With the first release of the resulting USNO Robotic Astrometric Telescope survey, URAT1, astronomers now have access to precise positions for about 228 million stars in the northern sky. That's a nearly fourfold increase in the star count per degree of sky.

Moreover, URAT1's star positions are accurate to between 5 and 30 milliarcseconds — equivalent to the apparent width of the "I" on a penny's "LIBERTY" as seen from 2 km (1¼ miles) away. These positional data are being combined with brightness measurements from two other big-sky efforts — 2MASS and the AAVSO's Photometry All-Sky Survey (APASS) — and eventually the USNO team will add parallax and proper-motion measurements of nearby stars.

URAT astrograph
This 0.2-meter astrograph at the U.S. Naval Observatory in Flagstaff, Arizona, took the 65,000 images used to compile the UCAC and URAT catalogs.

"The main goal of the URAT project was to provide a much denser, accurate reference star catalog going beyond UCAC," Zacharias explains. Plans were actually drawn up back in 2000, and the USNO team even dreamed of building a 0.85-m telescope to feed the camera. But the funds for the dedicated scope weren't available, so the Red Lens would have to do.

Even so, URAT1 catalog will be a boon to those trying determine the orbits of asteroids and comets or predicting stellar occultations by solar-system objects.

Zacharias says the USNO has just decided to return the astrograph to Chile, to capture the far-southern sky with the improved system. But it won't be probing down to 19th magnitude; instead, he says, the goal is to take a year to map out bright stars down to about 6th magnitude. So why not duplicate the very faint northern effort? In a word, Gaia. This spacecraft, launched by the European Space Agency in December 2013, is well on its way to mapping the positions of roughly a billion stars and other objects in our galaxy.

URAT1 sky coverage
This plot shows the sky coverage of the URAT sky catalog and the density of stars it's recorded per square degree of sky.

When URAT was conceived, Gaia was "science fiction — way into the future," notes Zacharias. A deep southern survey, reaching to about magnitude 18.5, wouldn't be completed until mid-2016 at the earliest — exactly when astronomers expect to get Gaia's first data release, with far more stars and positional data that's about 100 times more accurate. "So that would not make much sense," he says. Instead, URAT will concentrate on the southern stars that are 6th magnitude and brighter, the ones too dazzling for Gaia's detectors.

According to a USNO press release, the URAT1 catalog is available to users through the Strasbourg Astronomical Data Center's astrometric catalog server. Find out more about the project at this USNO website and this meeting poster.

You don't need a billion-star database to cruise the heavens from your backyard. A better bet is one of our top-quality star atlases.


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May 16, 2015 at 5:05 pm

One minor detail that Kelly missed. The new "4-shooter" camera consists of an array of four of the 10,560 X 10,560 110-million pixel CCDs. That's how we're able to get 28 square degrees of sky in one field.

Geoff Chester
USNO Public Affairs Office

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Peter Wilson

May 17, 2015 at 9:41 am

And with a dial-up modem, the entire catalog can be downloaded in less than a decade...

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May 18, 2015 at 11:21 am

how much time elapses before proper motion overwhelms the 0.005 arc-second precision?

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May 22, 2015 at 4:16 pm

For Barnard's Star, three days.

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